The global economy utilizes millions of tons per year of synthetic crystalline microporous inorganic zeolites for applications such as petrochemical cracking, ion-exchange for water softening and purification and in gas separations. Synthetic inorganic zeolites typically consist of oxide anions that link tetrahedral aluminum or silicon cations (nodes) in a 2:1 ratio.2 The key to the existence of microporosity in zeolites is that the oxide linkers are angular (M-O-M angles typically range from 140° to 165°, thereby facilitating the generation of a wide range of topologies that are based upon rings, fused rings and polyhedral cages. Which particular topology exists for a given chemical composition is typically controlled by reaction conditions, counterions and/or structure directing agents3 (SDAs). The absence of counterions, SDAs or the use of a linear linker more typically manifests the tetrahedral node in the form of a diamondoid (dia) net4,5 that, unlike most zeolitic topologies, can interpenetrate to mitigate the creation of free space.
The ground rules for generating zeolitic and/or diamondoid networks are therefore self-evident and they have been validated across a remarkably diverse range of tetrahedral nodes (e.g., phosphates,6 transition metal cations,7-9 metal clusters10) and linkers (including purely organic ligands that form coordination bonds11 or hydrogen bonds12-14). Coordination polymers that exploit the diversity of tetrahedral moieties and angular or linear organic ligands have recently afforded new levels of scale that includes new classes of zeolitic structures with hitherto unattainable levels of porosity. Such zeolitic metal-organic materials are exemplified by zeolitic imidazolate frameworks15-17 (ZIFs), boron imidazolate frameworks18 (BIFs) and zeolite-like metal-organic frameworks19 (ZMOFs). ZIFs are based upon imidazolate ligands that subtend an angle of ca. 145° whereas the prototypal ZMOFs use 4,5-imidazole-dicarboxylate20 and pyrimidine-based ligands21,22 in the presence of SDAs to coordinate to 8-coordinate metals such as In and Cd. BIFs are inherently of low density because they are based upon tetrahedral boron atoms. That low density is a desirable property means that lithium, the lightest metal in the periodic table, is a particularly attractive target to serve as a tetrahedral node in either zeolitic or dia networks. Furthermore, lithium forms many air and water stable coordination environments and not all existing zeolitic metal-organic materials are water-stable. In this context, a prototypal structure was reported by Pinkerton et al., who isolated a lithium-based zeolitic ABW network with hexachlorotantalum anion embedded in what was described as a three-dimensional Li—Cl-dioxane network.23 However, this compound is extremely moisture sensitive. Bu and coworkers addressed the challenge elegantly in BIFs by employing both lithium and boron with imidazolates in BIF-9-Li, a compound with RHO topology.24 Other approaches to low density porous materials based upon lithium include the following: Robson et al. reported a microporous lithium isonicotinate with square channels;25 Henderson and coworkers isolated a pillared bilayer and a diamondoid net with solvated lithium aryloxides;26 Parise et al. reported a MOF based on lithium and 2,5-pyridinedicarboxylic acid that loses porosity upon solvent removal.27 